This document discusses Java packages and JAR archives. It explains that packages are used to organize related classes and avoid naming collisions. Classes are organized into a directory structure that matches their package hierarchy. The classpath is used to locate classes and JAR files can be used to distribute applications and resources. The document provides examples of how to declare packages, import classes, and access resources within JAR files.

1. 1
CSE 331
Java Packages; JAR Archives
slides created by Marty Stepp
based on materials by M. Ernst, S. Reges, D. Notkin, R. Mercer, Wikipedia
http://www.cs.washington.edu/331/

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Java packages
• package: A collection of related classes.
 Can also "contain" sub-packages.
 Sub-packages can have similar names,
but are not actually contained inside.
•java.awt does not contain java.awt.event
• Uses of Java packages:
 group related classes together
 as a namespace to avoid name collisions
 provide a layer of access / protection
 keep pieces of a project down to a manageable size

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Packages and directories
• package  directory (folder)
• class  file
• A class named D in package a.b.c should reside in this file:
a/b/c/D.class
 (relative to the root of your project)
• The "root" directory of the package hierarchy is determined by your
class path or the directory from which java was run.

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Classpath
• class path: The location(s) in which Java looks for class files.
• Can include:
 the current "working directory" from which you ran javac / java
 other folders
 JAR archives
 URLs
 ...
• Can set class path manually when running java at command line:
 java -cp /home/stepp/libs:/foo/bar/jbl MyClass

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A package declaration
package name;
public class name { ...
Example:
package pacman.model;
public class Ghost extends Sprite {
...
}
• File Sprite.java should go in folder pacman/model .

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Importing a package
import packageName.*; // all classes
Example:
package pacman.gui;
import pacman.model.*;
public class PacManGui {
...
Ghost blinky = new Ghost();
}
• PacManGui must import the model package in order to use it.

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Importing a class
import packageName.className; // one class
Example:
package pacman.gui;
import pacman.model.Sprite;
public class PacManGui {
Ghost blinky = new Ghost();
}
• Importing single classes has high precedence:
 if you import .*, a same-named class in the current dir will override
 if you import .className, it will not

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Static import
import static packageName.className.*;
Example:
import static java.lang.Math.*;
...
double angle = sin(PI / 2) + ln(E * E);
• Static import allows you to refer to the members of another class
without writing that class's name.
• Should be used rarely and only with classes whose contents are
entirely static "utility" code.

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Referring to packages
packageName.className
Example:
java.util.Scanner console =
new java.util.Scanner(java.lang.System.in);
• You can use a type from any package without importing it if you
write its full name.
• Sometimes this is useful to disambiguate similar names.
 Example: java.awt.List and java.util.List
 Or, explicitly import one of the classes.

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The default package
• Compilation units (files) that do not declare a package are put into a
default, unnamed, package.
• Classes in the default package:
 Cannot be imported
 Cannot be used by classes in other packages
• Many editors discourage the use of the default package.
• Package java.lang is implicitly imported in all programs by default.
 import java.lang.*;

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Package access
• Java provides the following access modifiers:
 public : Visible to all other classes.
 private : Visible only to the current class (and any nested types).
 protected : Visible to the current class, any of its subclasses, and
any other types within the same package.
 default (package): Visible to the current class and any other types
within the same package.
• To give a member default scope, do not write a modifier:
package pacman.model;
public class Sprite {
int points; // visible to pacman.model.*
String name; // visible to pacman.model.*

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Package exercise
• Add packages to the Rock-Paper-Scissors game.
 Create a package for core "model" data.
 Create a package for graphical "view" classes.
 Any general utility code can go into a default package or into another
named utility (util) package.
 Add appropriate package and import statements so that the types can
use each other properly.

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• JAR: Java ARchive. A group of Java classes and supporting files
combined into a single file compressed with ZIP format, and given
.JAR extension.
• Advantages of JAR files:
 compressed; quicker download
 just one file; less mess
 can be executable
• The closest you can get to having a .exe
file for your Java application.
JAR Files (yousa likey!)

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Creating a JAR archive
• from the command line:
jar -cvf filename.jar files
 Example:
jar -cvf MyProgram.jar *.class *.gif *.jpg
• some IDEs (e.g. Eclipse) can create JARs automatically
 File → Export... → JAR file

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Running a JAR
• Running a JAR from the command line:
 java -jar filename.jar
• Most OSes can run JARs directly by double-clicking them:

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Making a runnable JAR
• manifest file: Used to create a JAR runnable as a program.
jar -cvmf manifestFile MyAppletJar.jar
mypackage/*.class *.gif
Contents of MANIFEST file:
Main-Class: MainClassName
 Eclipse will automatically generate and insert a proper manifest
file into your JAR if you specify the main-class to use.

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Resources inside a JAR
• You can embed external resources inside your JAR:
 images (GIF, JPG, PNG, etc.)
 audio files (WAV, MP3)
 input data files (TXT, DAT, etc.)
 ...
• But code for opening files will look outside your JAR, not inside it.
 Scanner in = new Scanner(new File("data.txt")); // fail
 ImageIcon icon = new ImageIcon("pony.png"); // fail
 Toolkit.getDefaultToolkit().getImage("cat.jpg"); // fail

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Accessing JAR resources
• Every class has an associated .class object with these methods:
 public URL getResource(String filename)
 public InputStream getResourceAsStream(String name)
• If a class named Example wants to load resources from within a
JAR, its code to do so should be the following:
 Scanner in = new Scanner(
Example.class.getResourceAsStream("/data.txt"));
 ImageIcon icon = new ImageIcon(
Example.class.getResource("/pony.png"));
 Toolkit.getDefaultToolkit().getImage(
Example.class.getResource("/images/cat.jpg"));
 (Some classes like Scanner read from streams; some like Toolkit read from URLs.)
 NOTE the very important leading / character; without it, you will get a null result

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JAR to EXE (JSmooth)
• JSmooth is a free program that
converts JARs into Windows EXE files.
 http://jsmooth.sourceforge.net/
 If the machine does not have Java
installed, your EXE will help the user
to download and install Java.
 A bit of a hack; not generally needed.
• Using JSmooth:
 choose Skeleton → Windowed Wrapper
 name your .exe under Executable → Executable Binary
 browse to your .jar under Application → Embedded JAR
 select the main class under Application → Main class